Machining small diameter, high l/d ratio parts “one shot”

[Ross Lloyd 1Participant @rosslloyd1]

Hi

I need to machine a small diameter but large l/d ratio part. My first attempt lead to a bent part I watched some videos on youtube, and the suggestion seems to be to start with a large piece of rigid stock, and then turn the part in "one shot" avoiding multiple passes to size. Now, this seems to be a solution that works well for the experienced lads on their nice expensive professional lathes, but will this work for a beginner on a Warco WM250V?

The setup is as below. 12mm stock in a collet chuck. Turning the first diameter, if its to be in "one shot", is a 4mm depth of cut (12mm down to 4mm), and the second would be 2.5mm (12mm down to 7mm). The aim of this one shot approach is to avoid subsequent cuts that exert sideways pressure on the part and hence bend it. But I am far from convinced that a WM250V is going to have the grunt for a 4mm DOC or that the 4way tool post has anything like the rigidity needed.

I've seen others using a special small diameter turning tool with little support bushings and taking multiple passes. However I am working from the Workshop Practice Series Lathe book, and the author just suggests chucking an appropriate sized piece of material in a 3 jaw and going for it as you would any other piece. As I already saw bending with a collet chuck, I feel like I am missing something.

Do I actually need a special approach for such a part, and if so is a WM250V going to choke with that depth of cut? What kind of tool and speeds might be recommended? Alternately, is there some other way to do this without having to buy / make an accessory?

Here's the video I am going from, see 5:03 for the kind of cut we are talking:

I started a conversation with Joe there (user: TheDroidBay) but I am too much of a newb to understand the reply! 

Edited By Ross Lloyd 1 on 30/12/2018 00:44:42

[Ross Lloyd 1Participant @rosslloyd1]

[HopperParticipant @hopper]

Turning a 4mm diameter by 25mm long should not be that hard to do. It's not a particularly long, thin job. Can you post some pics of the set up and tooling you are trying to use? A very sharp HSS toolbit might be your best bet. Less cutting force than inserts.

Othewise, you might try a moving steady that clamps to the carriage and moves just ahead of the cutting tool. I;m sure there must be one available for your model lathe.

Alternatively you could drill a small centre hole in the end of the job and support it with a tailstock centre, probably the the "half centre" type that has been ground away to allow tool access to small diameters. You could then cut the end off and face it afterwards, holding the job in a small sleeve in the three jaw chuck (or collet).

If you want to pursue the single cut idea, you might start with smaller diameter stock to reduce the depth of cut needed. You need to do a few test runs to determine the depth of cut your lathe is comfortable with. Stop looking at Youtube and get out there and just play with your lathe to see what it can do. Try different tools, speeds, set ups etc. (Remember, small diameters like this need high rpms to achieve the required 100 ft/min surface speed. Slow rpm will encourage dig ins etc. For 12mm diameter steel, your rpm should be about 800rpm with HSS toolbit. Double that for carbide insert tooling. Once you get down to 4mm diameter, rpm should be double that again, or more. Again, play with it and see what works best on your machine.

Another way of turning long slender jobs (which this is not really one of) is to do it in stages. IE, stick the job just a little out of the chuck and with progressive cuts turn it to size for say 8mm length, then slide the stock out of the chuck another 8mm and turn down the next 8mm length to size and so on.

Edited By Hopper on 30/12/2018 01:11:25

Edited By Hopper on 30/12/2018 01:13:15

[blowlampParticipant @blowlamp]

 

You could try one of these inserts that are designed for use with aluminium, they are very sharp and therefore have a lower tendency to bend long, thin parts. Use a 0.2mm radius tool to keep cutting forces low.

I would use 8mm stock with the minimum protruding from the chuck. Take as deep a cut as possible for a distance of say 10mm, for as many times as needed and take a finishing cut of around 0.1mm. Work your way along the bar in the same way until you reach the 25mm measurement. The aim being to use the rigidity of the stock to minimise deflection whilst cutting at the unsupported end.

Martin.

 

Edited By blowlamp on 30/12/2018 01:51:45

[Perko7Participant @perko7]

I've turned up various types of mild steel pins around that size in multiple passes with no problem. Start off with a 10mm bar and work down from there. You may need to run the last pass twice without adjusting the cross-slide to make sure the piece is the correct diameter for the full length. I assume it's not some expensive rare metal you are using, so if it bends then cut it off, drop it in the bin and start again. You'll soon get a feel for what you can achieve.

[Michael GilliganParticipant @michaelgilligan61133]

Ross

This recent thread [and particularly Stewart Hart's contribution] should help: **LINK**

https://www.model-engineer.co.uk/forums/postings.asp?th=139744

You're working at a larger scale, but the principles are relevant.

MichaelG.

Edited By Michael Gilligan on 30/12/2018 06:38:22

[Mike PooleParticipant @mikepoole82104]

I think the one shot cut is to avoid sneaking up on the final size which tends to push slender jobs away from the cutter rather than take a cut. What you need to know is how small you can go before the job loses the rigidity to cope with the cutting force. Getting the required surface finish from a cut that is going to be heavier than your normal finishing cut may need some testing to find the best speed and feed for your machine. It is probably a fair bet that getting it right first time requires experience of the tool, the machine and the material. I would think even a professional turner would need to explore a new machine to get the best from it.

Mike

[JasonBModerator @jasonb]

As hopper says 4mm x 25 long is not much of a challenge, just a sharp tool so the work does not get pushed away from the tool. For tipped then one of teh **GT inserts with 0.2mm corner radius should do it no problem.

Did this similar 5/32" diameter x 44 long valve stem the other day in stainless, roughing with a CCMT to 6mm dia then changed to the CCGT. Thread was done in one stage then the work moved further out of the collet to complete.

[Ross Lloyd 1Participant @rosslloyd1]

Thanks all for the replies, I am going to try the "gradually move the work out of the chuck" method so there is added support first, as this seems most straightforward and Jason's results do seem to speak for themselves. I don't have that style of collet, I have ER32, any reason why the method may not work with that?

Cheers as usual, replies here are always 1st class

[Chris TriceParticipant @christrice43267]

Don't forget, tailstock support is available.

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